416 MUSCLES AND FASCIA. 



its upper border, with the lower margin of the Pyriforrnis. By its lower border, 

 with the tendon of the Obturator Internus. 



The Gemellus Inferior arises from the upper part of the outer border of the 

 tuberosity of the ischium, and passing horizontally outwards, is blended with 

 the lower part of the tendon of the Obturator Internus, and inserted with it 

 into the upper border of the great trochanter. 



delations. By its -superficial surface, with the Gluteus Maximus, and the sciatic 

 vessels and nerves. By its deep surface, with the capsular ligament of the hip- 

 joint. By its upper border, with the tendon of the Obturator Internus. By 

 its lower border, with the tendon of the Obturator Externus and Quadratus 

 Femoris. 



The Quadratus Femoris is a short, flat muscle, quadrilateral in shape (hence 

 its name), situated between the Gemellus Inferior and the upper margin of the 

 Adductor Magnus. It arises from the outer border of the tuberosity of the 

 ischium, and proceeding horizontally outwards, is inserted into the upper part 

 of the linea quadrati, on the posterior surface of the trochanter major. A sy no- 

 vial bursa is often found between the under surface of this muscle and the lesser 

 trochanter, which it covers. 



Relations. By its posterior surface, with the Gluteus Maximus and the sciatic 

 vessels and nerves. By its anterior surface, with the tendon of the Obturator 

 Externus and trochanter minor, and with the capsule of the hip-joint. By 

 its upper border, with the Gemellus Inferior. Its lower border is separated from 

 the Adductor Magnus by the terminal branches of the internal circumflex 

 vessels. 



Dissection. In order to expose the next muscle (the Obturator Externus), it is necessary to 

 remove the Psoas, Iliacus, Pectineus, and Adductor Brcvis and Adductor Longus muscles from 

 the front and inner side of the thigh ; and the Gluteus Maximus and Qnadratas Femoris from 

 the back part. Its dissection should consequently be postponed until the muscles of the anterior 

 and internal femoral regions have been examined. 



The Obturator Externus (Fig. 255) is a flat triangular muscle, which covers 

 the outer surface of the anterior wall of the pelvis. It arises from the margin 

 of bone immediately around the inner side of the obturator foramen, viz., from 

 the body and ramus of the pubes, and the ramus of the ischium ; it also arises 

 from the inner two-thirds of the outer surface of the obturator membrane, and 

 from the tendinous arch which completes the canal for the passage of the obtu- 

 rator vessels and nerves. The fibres converging pass outwards and backwards, 

 and terminate in a tendon which runs across the back part of the hip-joint, and 

 is inserted into the digital fossa of the femur. 



Relations. By its anterior surface, with the Psoas, Iliacus, Pectineus, Adduc- 

 tor Longus, Adductor Brevis, and Gracilis; and more externally, with the neck 

 of the femur and capsule of the hip-joint. By its posterior surface, with the 

 obturator membrane and Quadratus Femoris. 



Nerves. The Gluteus Maximum is supplied by the inferior gluteal nerve and 

 a branch from the sacral plexus; the Gluteus Medius and Gluteus Minimus, by 

 the superior gluteal; the Pyriformis, Gemelli, Obturator Internus, and Quad- 

 ratus Femoris, by branches from the sacral plexus, and the Obturator Exter- 

 nus, by tlie obturator nerve. 



Actions. The Glutei muscles, when they take their fixed point from the 

 pelvis, are all abductors of the thigh. The Gluteus Maxinms and the posterior 

 libres of the Gluteus Medius, rotate the thigh outwards; the anterior fibres of 

 the Gluteus Medius and the Gluteus Minimus rotate it inwards. The Gluteus 

 Maximus serves to extend the femur, and the (ilnteus Medius and Gluteus 

 Minimus draw it forwards. The Gluteus Maximus i> also u tensor of the faseia 

 lata. Taking their fixed point from the femur, the Glutei muscles act upon 

 the pelvis, supporting it and the whole trunk upon the head of the femur, which 

 is especially obvious in standing on one leg. In order to gain the erect posture 

 after the effort of stooping, these muscles draw the pelvis backwards, assisted 



